The present invention relates to a wavelength tunable laser and an optical device using the wavelength tunable laser. More particularly, the invention relates to a semiconductor laser capable of tuning a lasing wavelength over a wide range, an optical modulator using the semiconductor laser, and a wavelength-division multiplexing transmission system employing, as a light source, a semiconductor laser used for a wavelength-division multiplexing optical system for multiplexing a plurality of different signal light and transmitting the multiplexed signal.
One of the important techniques in a wavelength-division multiplexing optical system is management of a wavelength of a light source of each of a plurality of channels. In the present optical communication systems, in order to maintain the wavelength of the light source at a predetermined value, a wavelength monitor and means for stabilizing the wavelength of the light source by feedback are provided for each channel and a spare light source prepared for a failure is provided for each of all of the channels. The number of related electronic devices therefore increases according to the number of channels. It is also necessary to control each semiconductor laser so that its lasing wavelength is within a predetermined narrow wavelength band. It is consequently difficult to improve the manufacturing yield. Such issues interfere with the attempt to achieve miniaturization and reduction in cost of an optical transmission system and are significant issues in the case of further narrowing the interval between waves of channels or the case of increasing the number of channels.
On the other hand, there is an idea such that the lasing wavelengths necessary for a plurality of channels are covered by a single backup light source by using a lasing wavelength tunable semiconductor laser. In this case, a wavelength tunable semiconductor laser capable of easily and successively sweeping the lasing wavelength is necessary, but has not been realized until now.
In particular, in an optical multiplexing transmission of a long distance, it is necessary to realize the system in a form that an optical modulator is monolithically integrated by which chirping can be reduced. In a monolithic integrated light source in which an optical modulator is incorporated, by adjusting the temperature of the whole light modulator, the wavelength of a channel can be adjusted. Presently, however, the operating temperature range of the monolithic integrated optical modulator is as narrow as xc2x15 degrees centigrade. The width of the wavelength which can be swept in practice is therefore only about 0.5 nm.
FIG. 9 shows the configuration in cross section of a wavelength tunable semiconductor laser capable of easily and successively sweeping the wavelength, in which a heater electrode is attached to a conventional buried heterostructure semiconductor laser. (For example, a technique described in IEEE Photonics Technology Letters, Vol. 4, p. 321, 1992 can be mentioned as a wavelength-division multiplexing system light source of this kind). According to the technique, a heater layer is formed over an upper electrode of a buried heterostructure semiconductor laser via an insulating film to control the temperature of an active layer. As shown by arrows with thick lines, since the heat generated by the heater layer escapes into not only the active layer but also a buried layer, the active layer cannot be efficiently heated. It is therefore a problem that the wavelength tuning efficiency, that is, the wavelength fluctuation range per unit power in the wavelength tunable semiconductor laser is as low as 3.2 nm/W.
It is therefore a main object of the invention to realize a wavelength tunable laser capable of tuning a wavelength over a wide range by simple means.
It is another object of the invention to realize a wavelength-division multiplexing transmission system which achieves the object and is suitable for a long distance transmission by using the wavelength tunable laser.
In order to achieve the objects, a wavelength tunable laser according to the invention is formed by mounting a thin film heater layer over and/or on a side of an upper electrode of a ridge waveguide semiconductor laser on a semiconductor substrate. The ridge waveguide semiconductor laser is obtained by forming a waveguide constructing a semiconductor laser in a ridge shape on a semiconductor substrate including a light emitting layer. The cross section of the ridge can have a shape of rectangle, trapezoid, or the like. An inverse trapezoid (inverse mesa) shape in which the side in contact with the semiconductor substrate is narrower than the upper side is preferable.
One of optical devices according to the invention is an integrated optical device in which the wavelength tunable laser and an external optical modulator are integrated on a semiconductor substrate of the wavelength tunable laser.
Further, another optical device according to the invention constructs a wavelength-division multiplexing transmission system for multiplexing light signal of a plurality of channels of different wavelengths and transmitting the light signal through a light transmission line. One or more wavelength tunable lasers are used as spare light source(s) of the plurality of light sources of the plurality of channels. When one of the light sources of the channels becomes faulty or the like and has to be replaced, the spare light source is allowed to operate and its wavelength is made coincide with the wavelength of the light source of the channel to be replaced by using the wave tuning function of the wavelength tunable laser.
The wavelength tunable laser of the invention enables the heat generated by the thin film heater to be efficiently applied to the light emitting part of the semiconductor laser. A monolithic integrated device is formed by combining the wavelength tunable laser with an optical modulator to thereby provide each of many optical devices such as a wavelength-division multiplexing transmission system with the effective means.